Adenylate kinase catalytic activity

M14. Matsuura, S., Igarashi, M Tanizawa, Y., Yamada, M., Kishi, F Kajii, T Fujii, H., Miwa, S Sakurai, M., and Nakazawa, A., Human adenylate kinase deficiency associated with hemolytic anemia. A single base substitution affecting solubility and catalytic activity of cytosolic adenylate kinase. J. Biol. Chem. 264, 10148-10155 (1989). 

A relative of the kinases is adenylate cyclase, whose role in forming the allosteric effector 3, 5 -cyclic AMP (cAMP) was considered in Chapter 11. This enzyme catalyzes a displacement on Pa of ATP by the 3 -hydroxyl group of its ribose ring (see Eq. 11-8, step a). The structure of the active site is known.905 Studies with ATPaS suggest an in-line mechanism resembling that of ribonuclease (step a, Eq. 12-25). However, it is Mg2+ dependent, does not utilize the two-histidine mechanism of ribonuclease A, and involves an aspartate carboxylate as catalytic base.906 All isoforms of adenylate cyclase are activated by the a subunits of some G proteins (Chapter 11). The structures907 of Gsa and of its complex with adenylate kinase905 have been determined. The Gsa activator appears to serve as an allosteric effector. 

The theoretical number of molecules of A converted to B per second would be 10. One molecule of epinephrine would result in the production of 10 G-GTP. Each activated a subunit would stimulate adenylate cyclase to produce 1000 cAMP molecules for a total of 10,000 molecules of cAMP. Each of these cAMP molecules would activate one catalytic subunit of protein kinase. (Remember that a molecule of protein kinase exists as an R2C2 complex. Two cAMP molecules combine with two R subunits to give two catalytically active C subunits.) Each of the 10,000 active C subunits would result in the production of 1000 molecules of active E, for a total of 10 molecules of active E. Each molecule of active E would in turn activate 1000 molecules of E2 for a total of 10 ° molecules of active E2. Since each molecule of active E2 would convert 1000 molecules of A to B per second, the total would be 1000 x 10 = 10 per second. (Note This is a greatly oversimplified example, but it illustrates the profound chemical amplification that can occur in systems under hormonal control.)... 

Figure 14-2. Regulation of cyclic AMP-dependent protein kinase A (PKA) by cyclic AMP. Activation of adenylate cyclase by binding of G( -GTP amplifies the signal by synthesis of many molecules of cyclic AMP. Cyclic AMP binding to PKA causes dissociation of the regulatory subunits from the catalytic subunits, which carry on the signal. Phosphodiesterase regulates the concentration of cyclic AMP by catalyzing its hydrolysis to AMP, which shuts off the signal.

The importance of the phosphoenzyme in the mechanism of action of succinyl-CoA synthetase in reactions (27a)-(27c) is also unknown. The mechanisms of action of aminoacyl-tRNA synthetases and of acyl-CoA synthetases do not include covalent enzymic intermediates. The fact that succinyl-CoA synthetase involves succinyl phosphate as the activated substrate, whereas the others involve acyl adenylates, does not explain the difference. There is no chemical catalytic basis for the mechanisms of the formation of these intermediates to vary in this way. Moreover, acetate kinase produces acetyl phosphate without the intermediate formation of a phosphoenzyme, so that at least acetate kinase has the capacity to catalyze direct phosphorylation of a carboxylate group. 

Some ligand-activated membrane receptors transmit their signal by stimulating adenylate cyclase activity in the cell to produce cAMP. This activation pathway is mediated by a receptor-associated G protein called GS (Chapter 16). In mammals, the most common mechanism by which cAMP serves as a second messenger involves cAMP binding to the regulatory subunit of cAMP-dependent protein kinase A (PKA). Dissociation of the regulatory subunit allows the catalytic sub-... 

Activation of adenylate cyclase by epinephrine or glucagon produces cAMP, which promotes the dissociation of cAMP-dependent protein kinase (R2C2) to give free catalytic monomers (C). C is the protein kinase, which phosphorylates other proteins. 

Adrenalin activates adenylate cyclase which synthesizes adenosine-3, 5 -cycUc monophosphate (cAMP), an activator of PrK. The enzyme (cAMP-dependent PrK) phosphorylates Ser and/or Thr (with consensus sequence of Arg-Arg-X-Ser/Thr-Y) of phosphorylase kinase consisting of C2R2. The binding of cAMP causes the dissociation of active catalytic monomers which utilizes ATP to phosphorylate phosphorylase b to the active phospho-phosphorylase a. The phosphorylation occurs at Serl4 of phosphorylase and requires Ca. The dephosphorylation of the active phospho form to the inactive dephospho form is catalyzed by PPrPl which becomes active when com-plexed with G-subunit The complexation of PPrPl(G) with its inhibitor releases phospho-(G) which is dephosphorylated to G-subunit by the action of PPrP2. 

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